1. Field of the Invention
This invention relates to a method and apparatus for injection molding by which a molding material, such as a macromolecular material, is formed into products having superior physical properties and a good appearance.
2. Description of the Related Art
Injection molding methods are widely used for processing a molding material which consists of or mainly includes a thermoplastic resin, as these methods have the advantage of a high productivity.
Recently, as a result of research into the physical properties of thermoplastic resins, it was found that the rigidity, heat-resistance and chemical properties of molded products are influenced by the molecular weight of the molding material, i.e., the thermoplastic resin.
The molecular weight of the thermoplastic resin which can be used in a conventional injection molding method, however, is relatively low, i.e., is lower than the molecular weight of the thermoplastic resin to be formed into a film, and therefore, the conventional injection molding method has a problem in that the physical properties such as rigidity, heat-resistance, and chemical properties of the molded products are unsatisfactory.
As means for improving these physical properties, several injection molding methods have been proposed by which the apparent fluidity of the resin is improved to increase the molecular weight thereof.
Japanese Examined Patent Publication No. 57-2088 proposed a method in which a gate of a mold is subjected to ultrasonic vibration. This method, however, has a problem in that the mold has a complex construction, and that the mold and the devices associated therewith are damaged by the ultrasonic vibration.
Japanese Unexamined Patent Publication No. 61-44616 proposed a method in which the surface of the mold is heated by induction heating under a high-frequency electromagnetic field, but as a result of experiments using this method, it was found that, even if the surface of the mold is heated, the expected improvement of the apparent fluidity of the resin is not obtained.
On the other hand, when a macromolecular material is used in injection molding, the material is heated while being fed into the mold. In this process, the material is thermally expanded by the heating process and then shrunk while in the mold in a cooling process, and thus only products having a size smaller than the size of the cavity of the mold can be obtained. Accordingly, to improve the dimensional accuracy of the molded products, several methods have been proposed for reducing the shrinkage of the molding material fed into the mold.
For example, it has been proposed that the injection molding pressure and clamping force be made much higher to enable a lowering of the processing temperature. This method, however, has a problem in that flaws occur in the molded product, and thus it is easily deformed in shape when used.
Japanese Unexamined Patent Publication No. 58-134722 proposed a method whereby the cavity to which the molding material is fed is formed by a horn for generating ultrasonic vibration, to thereby prevent fluctuations of the temperature of the material during the cooling process. This method, however, has a problem in that since the molding material is directly in contact with the horn, the load on the horn and a vibrator is too high, and thus it is difficult to transmit a required ultrasonic vibration to the molding material.